Passing a gas through a plasma can excite the gas and produce activated gases containing ions, free radicals, atoms and molecules. Activated gases and free radicals are used for numerous industrial and scientific applications including processing solid materials such as semiconductor wafers, powders, and other gases. Free radicals are also used to remove deposited thin films from semiconductor processing chamber walls.
Where activated gases or free radicals are used in processing, it may be desirable to preclude the plasma from interacting with the processing chamber or semiconductors being processed. Remote plasma sources can fill this need by generating the plasma, activated gases, and/or free radicals in a chamber that is isolated from the processing chamber, and then passing only the activated gases and/or free radicals to the processing chamber.
Plasmas can be generated in various ways, including DC discharge, radio frequency (RF) discharge, and microwave discharge. DC discharges are achieved by applying a potential between two electrodes in a gas. Plasmas generated via RF and DC currents can produce high-energy ions able to etch or remove polymers, semiconductors, oxides, and even metals. Therefore, RF or DC-generated plasmas are often in direct contact with the material being processed. Microwave discharges produce dense, low ion energy plasmas and, therefore, are often used to produce streams of activated gas for “downstream” processing. Microwave discharges are also useful for applications where it is desirable to generate ions at low energy and then accelerate the ions to the process surface with an applied potential.
Existing remote sources (e.g., toroidal and linear remote sources) have four main drawbacks. First, they fail to pull the plasma away from the remote source chamber walls thus allowing the plasma to etch the chamber walls. This will be referred to as poor plasma confinement. Second, they use a high power density to sustain the plasma, which generates high energy ions that bombard the remote source chamber walls and the processing chamber walls. Ion bombardment can also damage the wafers or other semiconductors being processed in the process chamber (e.g., etching low-k dielectrics). Third, toroidal and linear remote sources have significant electrostatic coupling to the plasma, which leads to further ion bombardment. Finally, these sources provide a narrow plasma cross-section through which non-activated or non-ionized gas can pass through. Thus, they may be limited in their effectiveness at dissociating non-activated gas.